1. Field of the Invention
The present invention relates to an epitaxial structure with increased piezoelectric effect, offering the possibility of integrating in the same semi-conductor material crystal one or more piezoelectric components and electronic components. The invention relates to the field of surface acoustic waves.
2. Description of the Prior Art
Surface acoustic waves are widely used in micro-electronics for constructing delay lines, filters adapted to coded signals or convoluters for example.
In general, an electronic system of this type is formed of two separate parts, one providing the acoustic function, the other the signal processing electronic function, each of the two parts being formed from a different material from the other part.
Elastic waves may treat electric signals in a frequency range extending from a few megahertz to several gigahertz, and the use of Rayleigh wave filters has provided a neat solution to the problem of filtering modern radar signals for example. But the structure of these filters is only simple if the substrate is piezoelectric. This requirement in practice restricts the number of useable materials: for example quartz (SiO.sub.2), lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3). To widen the choice of possible materials and functions obtainable, researches have been undertaken for depositing piezoelectric layers, for example aluminum nitrite (AlN) or zinc oxide (ZnO) on different substrates such as glass or alumina. Structures of the "ZnO layer on silicon substrate" kind allow hybrid integration.
For obvious reasons--the more thorough the integration the better the reliability--it is advantageous to integrate the acoustical electronic functions on a single substrate, a monolithic semi-conductor crystal allowing both functions to be provided at one and the same time. However, most of the materials known as semi-conductor materials, Si, GaAs, InP . . . etc, are not piezoelectric (Si) or only very slightly (Ga As, InP . . . ).
According to the invention, the piezoelectric effect of a material suitable for constructing a semi-conductor component but only slightly piezoelectric is increased by distorting its crystalline network, so as to increase the disymmetry thereof which is productive of the piezoelectric effect. The piezoelectricity of the material is thus increased, since it is forced by the desired distortion of its crystallographic network, which distortion is obtained by the disharmony of the crystalline mesh parameters between the substrate and the layer in which it is desired to provide the piezoelectric effect.
But, since the piezoelectric effect increased by the constraint is exerted over a small thickness, which will be discussed further on, and which does not exceed 200 .ANG. on average, and since on the other hand the surface waves move over a depth of the order of several thousand angstroms, the structures of the invention are formed by a plurality of layers of the increased piezoelectric effect material, alternating with a plurality of layers of the material of the substrate, or of the material having the same crystalline parameter, each interface between two successive layers, out of harmony in the mesh parameter, distorting the crystalline network of the piezoelectric layer.